I once tried finding some good rules for mass combat. But honestly, anything I found was just overly complicated for that aspect of the game.
It seemed to take the spotlight away from the PCs and I didn't want to do that. I also didn't see much difference in using the rules to determine outcomes of large battles or just rolling some generic rolls and adding in a few bonuses for each PC. The outcome will be the same (they either win or lose) and that outcome isn't really based on the individual actions of the PCs either way.
Instead, what I did was focus on the PCs experience in the battle and I described everything else going on around them. For example, I described that they are in the middle of a large battle, surrounded by enemies & allies both on the ground and in the sky. I described spells being cast and explosions going off around the battlefield. Then I ran a normal encounter with a select few enemies that the PCs fought. But I kept reminding them that tons of people were fighting around their little skirmish.
When they'd kill those enemies, I'd bring some more into the encounter as if the PCs were moving through the battlefield. Based on how their encounters played out (was it easy or difficult?), I gave bonuses or penalties to a couple of dice rolls to determine how successful their army was in the battle (or I'd just decide the outcome however I wanted).
This way, the PCs got to cast their spells or swing their weapons and drop enemies on their own. But they still had the impression that they were in a full scale war. I didn't have to learn new rules or teach them new rules. And at the end of the gaming day, they participated in a large battle just like they would have if I used mass combat rules.
I say, keep it simple and just wing it.